144 Structure and Development of the Skull in Sturgeons, [May 5, 



small temporary " head cavities," or remnants of the once continuous 

 subdivision of the body wall into an inner layer, the " splanchno- 

 pleure," and an outer layer, the " somato-pleure." 



Bat the aortic arches mount up, on each side, outside the proper 

 branchial arches, which become grooved to receive them ; these 

 arches must, therefore, be considered as developments of the tem- 

 porarily separate " splanchno-pleure " ; they cannot be classed with 

 the costal arches, which are developed in the permanently distinct 

 " somato-pleure." 



My dissections and sections, both of this type and of the Selachians, 

 show, without leaving room for doubt, that all the visceral, or, properly 

 speaking, branchial arches, mandibular, hyoid, and post-hyoid (bran- 

 chial proper) are developed in the outer walls of the large respiratory 

 pharynx, quite independently of the base of the skull and the fore 

 part of the spinal column. 



I have, at last, ceased to contend for true branchial or visceral 

 arches in front of the mouth, and also to look upon the mouth and the 

 openings around or in front of it as more than mere involutions of the 

 epiblast ; the first cleft is that between the mandible and the hyoid 

 arch, the first arch is the mandibular. 



With regard to the skull, it is now very evident that the " trabecules 

 cranii," even in their furthest growth forwards from the end of the 

 cephalic notochord, are merely foregrowths from the moieties of the 

 investing mass (the parachordals), the true axis of the cranial skeleton 

 ending under the fold of the mid-brain. The " cornua " of the trabeculse, 

 and the " intertrabecular " part or tract, are fresh shoots, so to speak, of 

 cartilage that are specially developed to finish the cranial box and the 

 internasal framework. I fear that my long-cherisbed pre-oral visceral 

 arches will now have to go down and take their place among these 

 secondary or adaptive growths. 



I may remark, in concluding this very imperfect abstract, that the 

 sturgeon is a very important type for the morphologist to get clear 

 light upon. 



In the Selachians the huge pterygoid foregrowth of the mandibular 

 arch aborts the apex of its pier, the function of which is supplied by 

 the hyo-mandibular ; fragments only are developed in its upper part. 



In the sharks from one to three mere " rays " are developed in front 

 of the small upper remnant of the first cleft ("spiracle ") ; in skates 

 there is, as a rule, a small separate piece, the true apex of the arch, 

 its " pedicle." In one kind, however, the torpedo, four such fragments 

 appear on each side, as shown by Gegenbaur. In the sturgeon there 

 is a most remarkable plate in the common metapterygoid region, its 

 form is rhomboidal ; ' it is composed of a number of well-compacted 

 pieces of cartilage, a middle series of azygous plates, and a some- 

 what irregular arrangement of plates right and left of these. This 



